1. Field of the Invention
The present invention relates to an ion exchange treatment for producing and regenerating an aqueous electric discharge machining fluid. More specifically, the present invention relates to a machining fluid generating method for die-sinking EDM or wire-cut EDM having a desired electrical conductivity upon ion exchange treatment.
2. Description of Related Art
In electric discharge machining for machining a metal workpiece by means of electric discharges, the electric discharge machining is performed in a dielectric medium. An aqueous EDM fluid, which is free from the danger of a fire, is increasingly used as the dielectric medium.
In die-sinking EDM, an aqueous solution, in which are dissolved water-soluble high molecular compounds for enhancing the electric discharge machining efficiency, may be used as an aqueous EDM fluid in place of an oil-based EDM fluid.
On the other hand, in wire-cut EDM for machining a workpiece into a desired configuration using a traveling wire electrode, which wire has been threaded into a start hole of the workpiece, in which electric discharges are induced between the wire and the workpiece while flushing the vicinity of the electric discharging point with a machining fluid, water is generally used as an aqueous EDM fluid because the workpiece may be placed in the atmosphere. Hereinafter, aqueous EDM fluid is referred to as "Machining Fluid" and aqueous EDM fluid is referred to as "Water Machining Fluid" when the aqueous EDM fluid is limited to wire-cut EDM.
Pure water (i.e., water in which almost all ions have been completely removed by ion exchange treatment) is ideal for a water machining fluid. In fact, water, whose electrical conductivity has been regulated to the range of from several .mu.S/cm to several tens of .mu.S/cm by admixing pure water with tap water, is generally used since it has several advantages, such as increased machining efficiency, low cost, and easy handling. The term "S" designates a siemens, a unit of conductance equal to 1 ampere per volt.
If the wire-cut EDM apparatus and the workpiece are constantly exposed to water containing chlorine ions, they tend to suffer from pitting corrosion (needle-like electrolytic corrosion) and rusting due to electrolytic activity. The pitting corrosion of the workpiece adversely affects the quality of the article being produced.
Consequently, it has been assumed that, in the case of admixing pure water with tap water to regulate electrical conductivity, only slight traces of chlorine ions in the mixed water would be acceptable. In general, when producing the water machining fluid, chlorine ions have been initially removed from the tap water before regulating electrical conductivity to a desired value.
The mixture of tap water and pure water, after used once in wire-cut EDM, may be discarded without regenerating it, but this is unprofitable in industrial practice because of increased running costs.
Therefore, generally, the water machining fluid used once in wire-cut EDM is regenerated. In this regenerating process, the once used water machining fluid is delivered to a storage tank and circulated through filtration equipment by a pump. Then, a portion of the filtered water machining fluid in the storage tank is further treated by ion exchange and returned to the storage tank. Additionally, when the volume of the machining fluid is low, because, for example, of evaporation thereof, supplementary tap water is replenished into the storage tank, and then ion exchange treatment is carried out until the overall electrical conductivity of the machining fluid in the storage tank reaches a value between several .mu.S/cm and several tens of .mu.S/cm, thus allowing reuse of the treated water as an electric discharge machining fluid.
The electrical conductivity of the water machining fluid is regulated to a range of from several .mu.S/cm to several tens of .mu.S/cm, because it has been assumed that there will be an increased risk of problems, such as an adherence of the Bs (brass) to the workpiece, if the conductivity is not maintained in this range, and because electrolytic corrosion is relatively less significant under such conditions, while maintaining high machining efficiency. In most cases, in treating tap water or used water machining fluid by ion exchange, raw water which has collected in the upper portion of a resin phase inside a resin column is allowed to flow down by the potential head. The resin phase in the resin column is formed such that the ratio of the height to the sectional area is large, in other words, it is taller than is wide.
Moreover, as the flow-down speed, a space velocity (referred to as "SV" herein) of about several h.sup.-1 is generally employed, because it has been assumed that in the conventional ion exchange treatment method, a large flow-down speed of raw water flowing down the resin phase would result in channeling, whereby ions in the raw water could leak into the treated water without being exchanged. The term "h.sup.-1 " designates the reciprocal of an hour, or "per hour".
The term "channeling" as used in the specification, means the phenomenon where raw water, by means of a short cut, passes through the resin phase.
Ion-exchange resins of high quality grade are used in producing pure water or a high-purity water used for semiconductor washing or in a nuclear power generation boiler. Similar resins are generally used also in the field of the electric discharge machining. Though such resins are suitable for adsorbing ionic substances mixed in the machining fluid as a result of electric discharge machining, which substances derive from elements which form a tool electrode and a workpiece, such resins are extremely expensive.
Used ion-exchange resins have been discarded without being regenerated by an acid or an alkali, since the metal particles produced during electric discharge machining are mingled in the used ion-exchange resins.
In conventional ion exchange treatment of used water machining fluid, almost all kinds of ions in raw water have been removed. Accordingly, there was a disadvantage that the treated water volume per unit volume of resins was decreased, namely, that the useful life of resins was shortened. In addition, since resins of high quality grade have only a small working capacity, there were disadvantages of increased running costs required for electric discharge machining and of frequent replacement of resins. Further, there was a disadvantage that, when continuously running an electric discharge machine in an unmanned factory over a long period of time, the ion-exchange resins could reach their working capacity limit during machining, thus degrading the article being produced. Thus, electric discharge machining has been commercially limited.